Inductor devices are extensively used in radio frequency integrated circuits (RFICs) and microwave monolithic integrated circuits (MMICs). Inductor devices are, for example, a primary component in inductance-capacitance voltage-controlled oscillator (LC-VCO) devices in these types of integrated circuits. An LC-VCO is an electronic oscillator specifically designed to be controlled in oscillation frequency by a voltage input. Typically, the one or more inductor devices in an LC-VCO are fabricated on the same substrate as the rest of the oscillator circuitry. For this reason, the area that an inductor device occupies on the integrated circuit and its ease of manufacture are major design considerations for the inductor device in addition to the inductor device's quality factor (Q-factor) and inductance value.
LC-VCOs in high-performance analog and mixed signal RFICs and MMICs are typically operated in a differential signal mode. In a differential signal mode, an LC-VCO produces two signals with a 180-degree phase difference. An important advantage of differential signal operation over single signal operation is its common mode rejection which results in greater immunity to environmental noise. In differential signal operation, two symmetric inductor devices are frequently used in a single LC-VCO. More recently, however, differential inductor devices have been utilized for differential signal operations. While comprising only a single winding, a differential inductor can be treated as two single-ended inductor devices that are symmetrically wound together. This winding together effectively causes the magnetic fluxes induced by the two out-of-phase signals to be added together (i.e. causes mutual inductance). Differential inductor devices therefore need about half of the central empty area (i.e., inductor core) required by a conventional inductor. Moreover, the Q-factor of a differential inductor device is typically significantly higher than that of a conventional inductor (i.e., up to about 50%) due to reduced interactions between the differential inductor device and the underlying semiconductor substrate.
Nevertheless, on-chip differential inductor devices have in the past typically been formed with specialized structures that are specific to the inductor devices and are not used in the remainder of the integrated circuit. For example, differential on-chip inductor devices have frequently required unconventionally thick metal lines in order to reduce their overall electrical resistance and improve their Q-factor. These specialized structures require additional processing over that required to form the remainder of the integrated circuit, adding considerable cost to the production of the integrated circuit. There is a need, as a result, for high Q-factor on-chip differential inductor device designs that can be produced in a conventional integrated circuit without the need for more processing steps than those required to form the remainder of the integrated circuit.